1. Field of the Invention
The present invention relates to a regulator circuit which stabilizes an input voltage for output.
2. Description of the Related Art
In some cases, to operate electronic circuits with stability, their supply voltages may be desirably stabilized at a constant value. Nevertheless, the supply voltage required by each electronic circuit is not always available in the apparatus in which the electronic circuits are incorporated. For example, a 5V microcomputer mounted in an automobile requires a supply voltage of 5V; however, the battery of the automobile supplies a voltage of 12V in an unstable manner. In such a case, a regulator circuit has been widely used to readily generate with stability the supply voltage required by the electronic circuit.
In general, the regulator circuit includes an error amplifier, an output transistor, and a feedback resistor. The error amplifier compares an output voltage fed back through the feedback resistor and a desired reference voltage to control the voltage at the control terminal of the output transistor so that the two voltages come close to each other. Accordingly, a variation in input voltage or load requires a change in voltage at the control terminal of the output transistor in response to the variation.
Here, in some cases, MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) would be used as the output transistor to reduce current consumption. When the MOSFET is used, an increase in size of the transistor to ensure a larger current capacity leads to an increase in its gate capacitance, thereby causing the gate voltage controlled by the error amplifier to respond to the variation in input voltage or load with delay. This delay in turn may cause an overshoot or undershoot in the output voltage. A load, i.e., a change in output current also causes an overshoot or undershoot.
To address such problems, a technique has been suggested in which a current flowing from the output transistor into the load is monitored to increase the bias current of the error amplifier according to the monitored current, thereby providing an improved response speed to the regulator (for example, see Japanese Patent Laid-Open Publication No. 2001-34351).
When a large amount of current flows through the load, using the technique described in the document above allows a large bias current to flow also through the error amplifier, thus improving the response speed. However, a sudden decrease in the current flowing through the load leads to a corresponding decrease in the response speed, thereby possibly causing the output voltage to vary. It is also difficult to suppress a variation in output voltage caused by a change in input voltage.